The present invention is directed to an article, such as an automobile body, that is electrocoated with a chip-resistant coating and a process for electrodepositing a chip-resistant coating on a substrate.
Multi-layered coating composites find use in various industries including the coating and/or painting of motor vehicles. In several of these industries, and in the automotive industry in particular, a substrate can have from two to six or more coating layers. These coating layers protect the substrate and provide a decorative finish.
Multi-layered coating composites for metal substrates often use electrodeposition coatings as an initial resinous coating layer to protect the metal substrate from corrosion. Cationic electrodeposition coatings have become the coatings of choice for corrosion protection. Electrodeposition has become increasingly important in the coatings industry because, by comparison with non-electrophoretic coating means, electrodeposition offers higher paint utilization, outstanding corrosion protection, low environmental contamination, and a highly automated process.
A spray-applied chip resistant coating layer is often present in multi-layered coating composites for motor vehicles. The chip resistant layer protects the surface of the substrates from losing paint through chipping when the vehicle is hit with solid debris, such as gravel and stones. The art for achieving chip resistance from spray applied primer coatings has postulated that reducing the differential in impact energy between the multiple coating layers should improve chip resistance of the coating. This is especially applicable when coating layers have excessive difference of hardness between them. This reduction in the differential would lessen delamination between the coatings, such as between an undercoat and an intermediate coat or between a top coat and an intermediate coat.
An object of the present invention is to provide a coating system having both good chip resistance and corrosion protection, while additionally providing efficiencies in application and processing through the elimination of a paint layer. These efficiencies include superior paint utilization, lower environmental contamination, and a highly automated process.
The present invention includes a coated article that comprises a corrosion-resistant electrically conductive substrate that is free of an electrodeposited coating and an electrodeposited film-forming composition applied to at least a portion of the surface of the corrosion-resistant electrically conductive substrate. The film forming composition includes a curable electrodepositable elastomeric polymer. The substrate typically is one of aluminum or alloys thereof, zinc or zinc alloy surface treated steel, such as galvanized steel, that is coated with a non-insulating layer of a zinc-rich or iron phosphide-rich organic coating. The present invention also includes a process for electrocoating a corrosion-resistant electrically conductive substrate which is free of an electrodeposited coating, including the step of electrodepositing a coating composition onto a surface of the electrically conductive substrate, the coating composition including a curable elastomeric polymer.
The corrosion-resistant substrate combined with an elastomeric electrodeposited coating performs as well or better than a less corrosion-resistant substrate coated with an electrodeposited corrosion-resistant primer followed by a spray-applied primer-surface layer. The above-described coating aids in 1) bonding the finished layer(s) to the substrate, 2) preventing chipping damage, 3) leveling defects in the substrate and providing a uniform under-layer, 4) protecting repairs and cut edges, and 5) resisting damage due to exposure to ultraviolet light.
The use of numerical values in the various ranges specified in this application, unless expressly indicated otherwise, are stated as approximations as though the minimum and maximum values within the stated ranges were both preceded by the word xe2x80x9cabout.xe2x80x9d In this manner slight variations above and below the stated ranges can be used to achieve substantially the same results as values within the ranges. Also, the disclosure of these ranges is intended as a continuous range including every value between the minimum and maximum values.
The present invention is directed to a coated article, typically an automobile body that includes a corrosion-resistant, electrically conductive substrate that has been coated with one or more layers of a film-forming composition. The corrosion-resistant substrate can be inherently corrosion-resistant, such as aluminum, or a substrate, such as a ferrous substrate, that has been rendered corrosion-resistant by application of a suitable pretreatment composition. The film-forming layer(s) include a layer that contains a curable, film-forming elastomeric polymer that is electrodeposited onto the corrosion-resistant, electrically conductive substrate. The elastomeric polymer preferably is a curable polyurethane, polyurea or poly(urethane-urea) that typically is the reaction product of a polyisocyanate and an active hydrogen-containing polymeric material. The active hydrogen-containing polymeric material preferably is a polyoxyalkylene ether polyol, a polyoxyalkylenepolyamine or a mixture thereof. The elastomeric polymer preferably contains a minimum amount of polyurea linkages when cured.
Corrosion-resistant conductive substrates include, without limitation, metal substrates such as stainless steel; zinc or zinc alloy surface treated steel, that is coated with a zinc-rich or iron phosphide-rich organic coating; aluminum; copper; magnesium or alloys thereof. By xe2x80x9csurface treatedxe2x80x9d it is understood that the surface of the steel has a zinc or zinc alloy layer that is introduced by a variety of known processes such as by, without limitation galvanizing or cladding the steel substrate. The substrate can be treated with a phosphating solution such as a zinc phosphating solution as described in U.S. Pat. No. 5,588,989. Combinations or composites of ferrous and/or non-ferrous metals also can be used, such as GALVALUME, GALVANNEAL and GALFAN zinc-aluminum alloys.
The terms xe2x80x9ccorrosion-resistant,xe2x80x9d and the like, refer to the relative resistance of the substrate to corrosion as compared to cold rolled steel. By xe2x80x9cnon insulatingxe2x80x9d it is meant that a coating does not interfere in any substantial way with the electrodeposition of the film-forming composition of the present invention (i.e., render the electrodeposited layer commercially unacceptable).
The pre-treatment composition also may be a solution that comprises one or more Group IIIB or IVB element-containing compounds or mixtures thereof solubulized or dispersed in a carrier medium, typically an aqueous medium. The Group IIIB and IVB elements are defined by the CAS Periodic Table of the Elements as shown, for example, in the Handbook of Chemistry and Physics, (60th Ed. 1980). Transition metal compounds and rare earth metal compounds typically are compounds of zirconium, titanium, hafnium, yttrium and cerium and mixtures thereof. Typical zirconium compounds may be selected from hexafluorozirconic acid, alkali metal and ammonium salts thereof, ammonium zirconium carbonate, zirconyl nitrate, zirconium carboxylates and zirconium hydroxy carboxylates such as hydrofluorozirconic acid, zirconium acetate, zirconium oxalate, ammonium zirconium glycolate, ammonium zirconium lactate, ammonium zirconium citrate, and mixtures thereof. Hexafluorozirconic acid is preferred. An example of a yttrium compound is yttrium nitrate. An example of a titanium compound is fluorotitanic acid and its salts. An example of a hafnium compound is hafnium nitrate. An example of a cerium compound is cerous nitrate. Preferably, the Group IIIB or IVB metal compounds are in the form of metal salts or acids which are water soluble. The Group IIIB or IVB metal compound is typically present in the carrier medium in an amount of 10 to 5000 ppm metal, preferably 100 to 300 ppm metal.
The pretreatment composition carrier also may contain a film-forming resin. Suitable resins include reaction products of one or more alkanolamines and an epoxy-functional material containing at least two epoxy groups, such as those disclosed in U.S. Pat. No. 5,653,823. Preferably, such resins contain beta-hydroxy ester, imide, or sulfide functionality, incorporated by using dimethylolpropionic acid, phthalimide, or mercaptoglycerine as an additional reactant in the preparation of the resin. Alternatively, the reaction product is that of the diglycidyl ether of Bisphenol A (commercially available from Shell Chemical Company as EPON 880), dimethylol propionic acid, and diethanolamine in a (0.6 to 5.0):(0.05 to 5.5:1) mole ratio. Other suitable resins include water soluble and water dispersible polyacrylic acids such as those as disclosed in U.S. Pat. Nos. 3,912,548 and 5,328,525; phenol-formaldehyde resins as described in U.S. Pat. No. 5,662,746, incorporated herein by reference; water soluble polyamides such as those disclosed in WO 95/33869; copolymers of maleic or acrylic acid with allyl ether as described in Canadian patent application 2,087,352; and water soluble and dispersible resins including epoxy resins, aminoplasts, phenol-formaldehyde resins, tannins, and polyvinyl phenols as discussed in U.S. Pat. No. 5,449,415.
In one embodiment, the pretreatment composition includes a film-forming resin that is present in an amount of 0.005% to 30% based on the total weight of the pretreatment composition, and the above-described Group IIIB or IVB metal compound in an amount of 10 to 5000, preferably 100 to 1000, ppm metal based on total weight of the pretreatment composition. The weight ratio of the resin to Group IIIB or IVB metal or metal compound is from 2.0 to 10.0, preferably 3.0 to 5.0, based on the Group IIIB or IVB metal.
Non-ferrous or ferrous substrates can be pretreated with a non-insulating layer of organophosphates or organophosphonates such as those described in U.S. Pat. Nos 5,294,265 and 5,306,526, which are incorporated herein by reference. Such organophosphate or organophosphonate pretreatments are available commercially from PPG Industries, Inc. under the trade name NUPAL(copyright). Application to the substrate of a non-conductive coating, such as NUPAL, typically is followed by the step of rinsing the substrate with deionized water prior to the coalescing of the coating. This ensures that the layer of the non-conductive coating is sufficiently thin to be non-insulating.
The pretreatment coating composition can further comprise surfactants that function as aids to improve wetting of the substrate. Generally, the surfactant materials are present in an amount of less than about 2 weight percent on a basis of total weight of the pretreatment coating composition. Other optional materials in the carrier medium include defoamers and substrate wetting agents.
Preferably, the pretreatment coating composition is essentially free of chromium-containing materials, i.e., the composition contains less than about 2 weight percent of chromium-containing materials (expressed as CrO3), and more preferably less than about 0.05 weight percent of chromium-containing materials. Examples of such chromium-containing materials include chromic acid, chromium trioxide, chromic acid anhydride, dichromate salts such as ammonium dichromate, sodium dichromate, potassium dichromate, and calcium, barium, magnesium, zinc, cadmium and strontium dichromate. Most preferably, the pretreatment coating composition is free of chromium-containing materials.
In a typical pre-treatment process, before depositing the pre-treatment composition upon the surface of the metal substrate, it is preferred to remove foreign matter from the metal surface by thoroughly cleaning and degreasing the surface. The surface of the metal substrate can be cleaned by physical or chemical means, such as by mechanically abrading the surface or cleaning/degreasing with commercially available alkaline or acidic cleaning agents which are well know to those skilled in the art, such as sodium metasilicate and sodium hydroxide. A non-limiting example of a preferred cleaning agent is CHEMKLEEN(copyright) 163, an alkaline-based cleaner commercially available from PPG Pretreatment and Specialty Products of Troy, Mich. Following the cleaning step, the metal substrate is usually rinsed with water in order to remove any residue. The metal substrate can be air-dried using an air knife, by flashing off the water by brief exposure of the substrate to a high temperature or by passing the substrate between squeegee rolls. The pretreatment coating composition can be deposited upon at least a portion of the outer surface of the metal substrate. Preferably, the entire outer surface of the metal substrate is treated with the pretreatment composition. The thickness of the pretreatment film can vary, but is generally less than about 1 micrometer, preferably ranges from about 1 to about 500 nanometers, and more preferably ranges from about 10 to about 300 nanometers.
When the substrate comprises an aluminum alloy it is particularly important to avoid over-cleaning the substrate with cleaning agents such as CHEMKLEEN(copyright) 163, and the like. It is postulated, without any intention of being bound by this theory, that the over cleaning of the substrate with these agents damages the surface of the substrate, resulting in the exposure of and/or the release of Fe and Cu, which adversely affects the quality of the chip-resistant layer. A typical non-damaging time range for the exposure of the substrate to these cleaning agents is from one to two minutes. Damage to the surface of the substrate can be monitored empirically by taking gloss readings of the subsequent baked electrocoated layer. For example, it has been determined that typical aluminum alloys should not be treated for more than two minutes with the commercially available CHEMKLEEN(copyright) CK166HP/CK171 product (PPG Pretreatment and Specialty Products of Troy Mich.).
The pretreatment coating composition is applied to the surface of the metal substrate by any conventional application technique, such as by spraying, immersion or roll coating in a batch or continuous process. The temperature of the pretreatment coating composition at application is typically about 10xc2x0 C. to about 85xc2x0 C., and preferably about 15xc2x0 C. to about 60xc2x0 C. The pH of the preferred pretreatment coating composition at application generally ranges from about 2.0 to about 5.5, and is preferably about 3.5 to about 5.5. The pH of the medium may be adjusted using mineral acids such as hydrofluoric acid, fluoroboric acid, phosphoric acid, and the like, including mixtures thereof; organic acids such as lactic acid, acetic acid, citric acid, sulfamic acid, or mixtures thereof, and water soluble or water dispersible bases such as sodium hydroxide, ammonium hydroxide, ammonia, or amines such as triethylamine, methylethyl amine, or mixtures thereof.
Continuous processes typically are used in the coil coating industry and also for mill application. The pretreatment coating composition can be applied by any of these conventional processes. For example, in the coil industry, the substrate typically is cleaned and rinsed and then contacted with the pretreatment coating composition by roll coating with a chemical coater. The treated strip is then dried by heating, painted and baked by conventional coil coating processes.
Mill application of the pretreatment composition can be by immersion, spray or roll coating applied to the freshly manufactured metal strip. Excess pretreatment composition is typically removed by wringer rolls. After the pretreatment composition has been applied to the metal surface, the metal can be rinsed with deionized water and dried at room temperature or at elevated temperatures to remove excess moisture from the treated substrate surface and cure any curable coating components to form the pretreatment coating. Alternately, the treated substrate can be heated at about 65xc2x0 C. to about 125xc2x0 C. for about 2 to about 30 seconds to produce a coated substrate having a dried residue of the pretreatment coating composition thereon. If the substrate is already heated from the hot melt production process, no post application heating of the treated substrate is required to facilitate drying. The temperature and time for drying the coating will depend upon such variables as the percentage of solids in the coating, components of the coating composition and type of substrate.
The film coverage of the residue of the pretreatment composition generally ranges from about 1 to about 10,000 milligrams per square meter (mg/m2), and is preferably about 10 to about 400 mg/m2.
A layer of a weldable primer also may be applied to the substrate, whether or not the substrate is pretreated. Preferably the substrate is pretreated. A typical weldable primer is BONAZINC(copyright), a zinc-rich mill applied organic film-forming composition, which is commercially available from PPG Industries, Inc., Pittsburgh, Penn. BONAZINC(copyright) typically is applied to a thickness of at least/micrometer and preferably to a thickness of about 34 micrometers. Other weldable primers, such as iron phosphide-rich primers, are commercially available.
The chip-resistant film coating of the present invention is then applied to the corrosion-resistant electrically conductive substrate. The coating is electrodeposited from a composition that includes an ionic, elastomeric polymer, typically a polyurea, a polyurethane or a poly(urethane-urea). The elastomeric polymer typically is the reaction product of a polyisocyanate and an active hydrogen-containing polymer that is one of a polyester, a polyether, an acrylic polymer or combinations thereof. For example, the elastomeric polymer can be the reaction product of a polyisocyanate and an active hydrogen-containing polymer that is a polyester or polyether, preferably a polyoxyalkylene ether polyol, a polyoxyalkylenepolyamine or a mixture thereof. A suitable active hydrogen-containing polymer has a Tg of less than 0xc2x0 C. and a number average molecular weight of 400-4000. Examples of suitable active hydrogen-containing polymer (polyols) are described in U.S. Pat. No. 4,154,891. The Tg for many polymers, including polyethers are available in the literature and/or are determinable by well-known methods, such as by differential scanning calorimetry. Also helpful in determining the Tg is the Clash-Berg method (ASTM D-1043, a method for determination of the stiffness characteristics of plastics over a wide temperature range by direct measurement of the apparent modulus of rigidity). The elastomeric polymer typically has a flexibilizing segment content of from 30 to 100, preferably from 40 to 60. The flexibilizing segment content is determined in accordance with Example 1 part D.
The elastomeric polymer is ionic to permit electrodeposition of a resin formed therefrom. The ionic resin can be anionic or cationic, but preferably is cationic, the composition including the ionic resin being cationically electrodepositable. The elastomeric polymer typically has a weighted average molecular weight (Mw) of less than 100,000, preferably less than 50,000 and most preferably from 10,000 to 40,000. The elastomeric polymer also typically has active hydrogen functionality, i.e., hydroxyl, primary or secondary amine, and typically has an active hydrogen equivalent weight of 800 to 2,500 grams per equivalent. The molecular weight of the elastomeric polymer and other polymeric materials used in the practice of the invention is determined by gel permeation chromatography using a polystyrene standard. The elastomeric polymer preferably is essentially aliphatic to render the polyurethane or polyurea less susceptible to damage by UV radiation. By xe2x80x9cessentially aliphaticxe2x80x9d it is meant that the backbone of the elastomeric polymer contains no more than 50% by weight aromatic groups or moieties (i.e., the total weight of multivalent phenyl groups, such as phenylene ((Mw=76), in the backbone)), preferably no more than 20% by weight aromatic groups, most preferably no more than 5% by weight aromatic groups and optimally  less than 1% by weight aromatic groups. The percentage by weight of aromatic groups (A %) can be determined by the following formula:
A %=Wa/WtX100%
where Wa is the weight of aromatic groups in the aromatic group-containing reactant, which is the weight of the aromatic group-containing reactant multiplied by the fraction of the total weight of the aromatic group-containing reactant that is aromatic, which can be calculated as follows:
(76xc3x97the number of aromatic rings in the aromatic group-containing reactant)/molecular weight of the aromatic group-containing reactant.
Wt is the total weight of the reactants less the total weight of materials lost to volatilization during the synthesis and cure of the polymer. If more than one reactant contains aromatic groups, A % for each reactant can be added (i.e., A %1+A %2=A %total)
The elastomeric polymer typically is a polyurethane, a polyurea or a poly(urethane-urea). The elastomeric polymer preferably includes a minimum amount of urea linkages in its cured state. Typically, the minimum amount of urea linkages are greater than about 0.50 meq/g of cured film, preferably greater than about 0.60 meq/g of cured film and most preferably greater than about 1.00 meq/g of cured film. The meq/gram can be determined by determining the total meqs of urea linkages formed divided by the total weight in grams of the reactants used in preparing the polymer minus the weight of molecules lost, for example, by volatilization, in curing the film. For example, a polymer prepared from reactants containing 2 equivalents of isocyanate, 1 equivalent of amine and 1 equivalent of hydroxyl groups in 1000 grams of reactants would contain 1 equivalent of urea linkages and 1 equivalent of urethane linkages per 100 grams of cured film (assuming no loss of volatile molecules), which is the equivalent of 1 meq of urea linkages per gram of cured film. In practice, it is found that films that contain a minimum percent of urea linkages are more stable under typical baking (curing) temperatures, as compared to urethane linkages, which are less tolerant of overbaking.
Polyurethanes and polyureas typically are produced by reacting a polyisocyanate and an active hydrogen-containing polymer. Suitable polyisocyanates useful for preparing the elastomeric polymer include those that have aliphatically-, cycloaliphatically-, araliphatically- and/or aromatically-bound isocyanate groups. Preferably, the polyisocyanate is aliphatic or cycloaliphatic.
Examples of aliphatic and cycloaliphatic polyisocyanates include 4,4-methylenebisdicyclohexyl diisocyanate (hydrogenated MDI), hexamethylene diisocyanate (HDI)(i.e., 1,6-hexamethylene diisocyanate), isophorone diisocyanate (IPDI), methylenebis(cyclohexyl isocyanate), cyclohexylene diisocyanate (hydrogenated XDI) and isocyanurates thereof.
Examples of aromatic polyisocyanates include tolylene diisocyanate (TDI) (i.e., 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate or a mixture thereof), diphenylmethane-4,4-diisocyanate (MDI), naphthalene-1,5-diisocyanate (NDI), 3,3-dimethyl-4,4-biphenylene diisocyanate (TODI), crude TDI (i.e., a mixture of TDI and an oligomer thereof), polymethylenepolyphenyl polyisocyanate, crude MDI (i.e., a mixture of MDI and an oligomer thereof), xylylene diisocyanate (XDI) and phenylene diisocyanate.
Polyisocyanate derivatives prepared from hexamethylene diisocyanate, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane, including isocyanurates thereof, and/or 4,4xe2x80x2-bis(isocyanatocyclohexyl)methane are suitable.
The amount of polyisocyanate used to make the elastomeric polymer is typically from 10 to 60, preferably 20 to 50 percent by weight based on total weight of the reactants used to make the elastomeric polymer.
The active hydrogen-containing material for reaction with the polyisocyanate to form the cationic or anionic polyurethane comprises one or more active hydrogen-containing polymers. These materials preferably have an average active hydrogen functionality ranging from about 2 to 8, preferably from about 2 to 4, a number average molecular weight ranging preferably from about 400 to 10,000, more preferably from 400 to 4,000, and a glass transition temperature (Tg) of 0xc2x0 C. or less. Polyether polyols or polyamines are preferred. Polyether polyamines are most preferred.
The term xe2x80x9cactive hydrogenxe2x80x9d means those groups which are reactive with isocyanates as determined by the Zerewitnoff test as is described in the Journal Of The American Chemical Society, Vol. 49, page 3181 (1927). Preferably, the active hydrogens are hydroxyl, primary amine and secondary amine. Most preferably, the active hydrogens are amines.
Examples of polyether polyols are polyalkylene ether polyols that include those having the following structural formula: 
where the substituent R is hydrogen or lower alkyl containing from 1 to 5 carbon atoms including mixed substituents, and n is typically from 5 to 200. Included are, without limitation, poly(oxytetramethylene) glycols, poly(oxyethylene) glycols, poly(oxy-1,2-propylene) glycols and the reaction products of ethylene glycol with a mixture of 1,2-propylene oxide and ethylene oxide. These materials are obtained by the polymerization of alkylene oxides such as ethylene oxide, propylene oxide and tetrahydrofuran.
Also, polyethers obtained from the oxyalkylation of various polyols, for example, diols such as 1,6-hexanediol or higher polyols such as trimethylolpropane and sorbitol, can be used. One commonly utilized oxyalkylation method is the reacting of a polyol with alkylene oxide such as ethylene or propylene oxide in the presence of an acidic or basic catalyst.
Examples of polyamines are polyoxyalkylene polyamines. The preferred polyamines are polyoxyalkylene polyamines of the structure: 
wherein R can be the same or different and is selected from the class consisting of hydrogen, lower alkyl radicals having from 1 to 6 carbon atoms, and n represents an integer of from about 1 to 50, preferably 1 to 35. A number of such polyoxyalkylene polyamines are described in more detail in U.S. Pat. No. 3,236,895, column 2, lines 40-72; methods of preparation of the polyoxyalkylene polyamines are illustrated in the patent in Examples 4, 5, 6 and 8-12 in columns 4 to 9 thereof; the aforementioned portions of U.S. Pat. No. 3,236,895 hereby being incorporated by reference.
Mixed polyoxyalkylene polyamines can be used, that is, those in which the oxyalkylene group can be selected from more than one moiety. Examples are polyoxyethylene-propylene polyamines such as those having the following structural formula: 
wherein n+m is equal to 1 to 50, preferably 1 to 35, m is equal to 1 to 49, preferably 1 to 34, and n is equal to 1 to 34.
Besides the polyoxyalkylene polyamines mentioned above, derivatives of polyoxyalkylene polyamines or polyols also may be usable. Examples of suitable derivatives are aminoalkylene derivatives which are prepared by reacting polyoxyalkylene polyamines or polyols such as those mentioned above with acrylonitrile followed by hydrogenation of the reaction product. An example of a suitable derivative is that of the following structural formula: 
wherein X is O or NH, Y is a C1-6 alkylene group and n is 1-50.
In one particular embodiment, the polyoxyalkylene polyamine is polyoxytetramethylene diamine, which typically is the hydrogenation product of cyanoethylated polyoxytetramethylene diol (obtained from polyoxytetramethylene diol and acrylonitrile).
Therefore, in the practice of the invention, where the expression xe2x80x9cpolyoxyalkylene polyaminesxe2x80x9d is used, what is intended are polyamines containing both oxyalkylene groups and at least two amine groups, preferably primary amine groups, per molecule. Primary amines are considered to be monofunctional when reacting with isocyanate groups to form a urea linkage.
Preferred active hydrogen-containing polyethers are polyoxytetramethylene diols or diamines, such as polytetrahydrofuran and polyoxypropylene diamine or mixed polyoxyethylene-propylene diamines. Polyoxypropylene diamine and polyoxytetramethylene diamine are most preferred.
The polyether segment can be derived from a single polyether polyol or polyamine or various blends thereof. Preferred are blends of polyether polyols such as polyoxytetramethylene diol and polyether polyamines such as polyoxypropylene diamine polyoxytetramethylene diamine or in weight ratios of 0.5-10:1, preferably 1 to 7:1.
Besides active hydrogen-containing polyethers, other active hydrogen-containing materials can be reacted with the polyisocyanate to provide the elastomeric segment. To produce polyurethanes, examples include polycarbonate diols, polyester diols, hydroxyl-containing polydiene polymers, hydroxyl-containing acrylic polymers, and mixtures thereof. To produce polyureas, examples include without limitation, polycarbonated amines, polydienediamines and iminated polyester.
Examples of polyester polyols and hydroxyl-containing acrylic polymers are described in U.S. Pat. Nos. 3,962,522 and 4,034,017, respectively. Examples of polycarbonate polyols are described in U.S. Pat. No. 4,692,383 in col. 1, line 58 to col. 4, line 14. Examples of hydroxyl-containing polydiene polymers are disclosed in U.S. Pat. No. 5,863,646, col. 2, lines 11-54. These polymeric polyols will have number average molecular weights of from 400 to 10,000, preferably from 400 to 3,000.
The amount of the active hydrogen-containing polymer that is used to prepare the elastomeric polymer is at least 30 percent by weight, preferably at least 35 percent by weight, and more preferably from 35 to 50 percent by weight based on total weight of the reactants used to make the elastomeric polymer.
Low molecular weight active hydrogen-containing materials, such as polyols or polyamines having two to four hydroxyl or amine groups and molecular weights of less than 400, preferably less than 250 and most preferably between 62 and 250, also can be included as reactants for preparing the polyurethane. Specific examples include, without limitation, alkylene diols having 1 to 10 carbon atoms such as ethylene glycol, 1,2-propylene glycol, 1,4-butanediol, ethylene diamine, hexamethylene diamine, trimethylolpropane, glycerol, pentaerythritol and sorbitol. Examples of other low molecular weight polyols are ether polyols such as diethylene glycol and ethoxylated Bisphenol A.
The low molecular weight polyols or polyamines are used in amounts up to 30 percent by weight, preferably from 0.5 to 10 percent by weight based on weight of the reactants used to make the elastomeric polymer.
The elastomeric polymer formed from the reaction of the polyisocyanate and the active hydrogen-containing material typically contains ionizable groups, which can be ionized to solubilize the polymer in water, or groups can be reacted with an ionizable material to form an ionizable group. For the purposes of this invention, the term xe2x80x9cionizablexe2x80x9d means a group capable of becoming ionic, i.e., capable of dissociating into ions or becoming electrically charged. The ionizable moiety can be provided as a group present on the active hydrogen-containing material or as part of a second active hydrogen-containing material that is co-reacted with, or attached to, the active hydrogen-containing compound and the polyisocyanate. A preferred embodiment is a cationic polyurea that is substantially free of polyurethane linkages.
For the cationic polymers, the ionizable moiety typically is a tertiary amine group that can be incorporated into the polymer by reaction of the polymer with a co-reactive material containing the ionizable moiety (the tertiary amine group). The amine is neutralized with acid to form an amine salt group. Such compounds include aminoalcohols such as dimethylaminoethanol, dimethylaminopropanol, aminopropyldiethanolamine, diethylaminopropylamine, hydroxyalkylmorpholine like hydroxyethylmorpholine, and hydroxyalkylpiperazine like hydroxyethylpiperazine, and the like and mixtures thereof. The amount of amine introduced into the polymer typically is that sufficient to give 0.1 to 1, preferably 0.2 to 0.5 milliequivalents (meqs) of amine per gram of resin solids as determined by titration.
In a further embodiment, the elastomeric polymer can be prepared to include pendant epoxy groups, i.e., by the use of glycidol as an active hydrogen-containing compound, which can be reacted with a tertiary amine in the presence of an organic acid to form a quaternary amonium group or with a sulfide, such as thiodiethanol, in the presence of an organic acid to form a sulfonium group. A method for forming quaternary amonium groups is described in U.S. Pat. No. 5,908,912. A method for forming sulfonium groups is described in U.S. Pat. Nos. 3,793,278 and 3,959,106.
For the anionic polymer, the ionizable moiety is an acid group which typically is incorporated into the polymer by reaction with an active hydrogen-containing compound. The acid is neutralized with base to form the anionic salt group. Examples of anionic groups are xe2x80x94OSO3xe2x88x92, xe2x80x94COOxe2x88x92, xe2x80x94OPO3xe2x95x90, xe2x80x94SO2Oxe2x88x92, xe2x80x94POOxe2x88x92; and PO3xe2x95x90, with COOxe2x88x92 being preferred.
Suitable materials for introducing acid groups into the anionic polymer are hydroxy and mercapto carboxylic acids. Specific examples include dimethylolpropionic acid, which is preferred, glycolic acid and lactic acid. Other examples of compounds which contain active hydrogens and acid groups are amino carboxylic acids, amino hydroxy carboxylic acids, sulfonic acids, hydroxy sulfonic acids and amino sulfonic acids. Examples include oxaluric acid, anilido acetic acid, glycine, 6-amino-caprylic acid, reaction product of ethanolamine and acrylic acid, hydroxyethylpropionic acid, 2-hydroxy-ethane sulfonic acid and sulphanilic acid. Amino acids must be used in the presence of base such as potassium hydroxide or a tertiary amine. The amount of acid incorporated into the polymer typically is that sufficient to give the polymer 0.1 to 1, preferably 0.2 to 0.5 meqs of acid per gram of resin solids as determined by titration.
The amine or the acid groups are neutralized with acid and base, respectively. Neutralization can range from 0.1 to 1.0, preferably 0.4 to 0.8, of the total theoretical neutralization equivalent.
For the cationic polymers, suitable neutralizing agents are organic acids such as acetic acid, boric acid, hydroxyacetic acid, propionic acid, lactic acid, formic acid, tartaric acid, sulfamic acid and dimethylolpropionic acid that is preferred, as well as inorganic acids such as sulfuric acid, hydrochloric acid and phosphoric acid.
For anionic polymers, suitable neutralizing agents include inorganic and organic bases such as sodium hydroxide, potassium hydroxide, ammonia and amines. Suitable amines include alkanolamines such as monoethanolamine, diethanolamine, dimethylaminoethanol, triethanolamine, diisopropanolamine, triisopropanolamine, and the like; alkylamines such as diethylamine, triethylamine, diisopropylamine, tributylamine and the like; trimethylamine, diethylmethylamine, methyldiethanolamine, triethanolamine, and the like. The appropriate amount of the neutralizing agent is about 0.1 to 1.0 times, preferably 0.4 to 0.8 times the total theoretical neutralization equivalent.
To achieve optimum chip resistance and durability, the elastomeric polymer is curable, or thermosetting. As such, the polymer is electrodeposited with a curing or crosslinking agent such as a capped or blocked isocyanate, which is preferred for cationic compositions, or an aminoplast, which is preferred for anionic compositions.
The polyisocyanate may be fully capped with essentially no free isocyanate groups and present as a separate component or it may be partially capped and reacted with active hydrogen-containing groups, such as hydroxyl or amine groups in the elastomeric polymer backbone. Examples of suitable polyisocyanates and capping agents are those described in U.S. Pat. No. 3,947,339. Fully capped polyisocyanates are described in U.S. Pat. No. 3,984,299.
The polyisocyanate can be an aliphatic, cycloaliphatic or an aromatic polyisocyanate or a mixture of the two. Diisocyanates are preferred, although higher polyisocyanates can be used in place of, or in combination with, diisocyanates. Aliphatic or cycloaliphatic polyisocyanates are preferred.
Examples of suitable aliphatic diisocyanates are straight chain aliphatic diisocyanates such as 1,4-tetramethylene diisocyanate and 1,6-hexamethylene diisocyanate. Also, cycloaliphatic diisocyanates can be employed. Examples include isophorone diisocyanate and 4,4xe2x80x2-methylene-bis-(cyclohexyl isocyanate). Examples of suitable aromatic diisocyanates are p-phenylene diisocyanate, diphenylmethane-4,4xe2x80x2-diisocyanate and 2,4- or 2,6-toluene diisocyanate. Examples of suitable higher polyisocyanates are triphenylmethane-4,4xe2x80x2,4xe2x80x3-triisocyanate, 1,2,4-benzene triisocyanate and polymethylene polyphenyl isocyanate. Biurets and isocyanurates of diisocyanates, including mixtures thereof, such as the isocyanurate of hexamethylene diisocyanate, the biuret of hexamethylene diisocyanate, and the isocyanurate of isophorone diisocyanate are also suitable.
Isocyanate prepolymers, for example, reaction products of polyisocyanates with polyols such as neopentyl glycol and trimethylol propane or with polymeric polyols such as polycaprolactone diols and triols (NCO/OH equivalent ratio greater than one) can also be used.
Any suitable aliphatic, cycloaliphatic, or aromatic alkyl monoalcohol or phenolic compound may be used as a capping agent for the capped polyisocyanate crosslinking agent in the composition of the present invention including, for example, lower aliphatic alcohols such as methanol, ethanol, and n-butanol; cycloaliphatic alcohols such as cyclohexanol; aromatic-alkyl alcohols such as phenyl carbinol and methylphenyl carbinol; and phenolic compounds such as phenol itself and substituted phenols wherein the substituents do not affect coating operations, such as cresol and nitrophenol. Glycol ethers also may be used as capping agents. Suitable glycol ethers include ethylene glycol butyl ether, diethylene glycol butyl ether, ethylene glycol methyl ether and propylene glycol methyl ether.
Other suitable capping agents include oximes such as methyl ethyl ketoxime, acetone oxime and cyclohexanone oxime, lactams such as epsilon-caprolactam, and amines such as dibutylamine.
Examples of other blocked polyisocyanates include triazine compounds having the formula C3N3(NHCOXR)3, wherein X is nitrogen, oxygen, sulfur, phosphorus, or carbon, and R is an alkyl group having one to twelve, preferably one to four, carbon atoms, or mixtures of such alkyl groups. X is preferably oxygen or carbon, more preferably oxygen. R preferably has one to eight carbon atoms, for example, methyl, ethyl, n-propyl, isopropyl, butyl, n-octyl and 2-ethylhexyl. R preferably is a mixture of methyl and butyl groups. Such compounds, and the preparation thereof, are described in detail throughout U.S. Pat. No. 5,084,541, incorporated herein by reference. Examples of triazine compounds are tris carbamoyl triazine or 1,2,5 triazine-2,4,6 tris-carbamic acid esters. When used, the triazine curing agent is present in the film-forming composition in an amount ranging from 1 to 40 eight percent, preferably from 1 to 20 weight percent, more preferably 1 to 10 percent by weight based on the total weight of resin solids in the film-forming composition.
The polyisocyanate crosslinking agent typically is present in the thermosetting compositions of the present invention in an amount of at least 10 percent by weight, preferably at least 15 percent by weight, based on total resin solids weight of the composition. The polyisocyanate crosslinking agent typically is also present in the composition in an amount of less than 60 percent by weight, preferably less than 50 percent by weight, and more preferably less than 40 percent by weight, based on total resin solids weight of the composition. The amount of crosslinling agent present in the thermosetting composition of the present invention may range between any combination of these values, inclusive of the recited values.
The equivalent ratio of hydroxyl groups in the polymer to reactive functional groups in the crosslinking agent is typically within the range of 1:0.5 to 1.5, preferably 1.0 to 1.5.
Aminoplasts typically are obtained from the reaction of formaldehyde with an amine or amide. The most common amines or amides are melamine, urea, or benzoguanamine, and are preferred. However, condensates with other amines or amides can be used; for example, aldehyde condensates of glycoluril, which give a high melting crystalline product which is useful in powder coatings. While the aldehyde used most often is formaldehyde, other aldehydes such as acetaldehyde, crotonaldehyde, and benzaldehyde may be used.
The aminoplast contains methylol groups and preferably at least a portion of these groups are etherified with an alcohol to modify the cure response. Any monohydric alcohol may be employed for this purpose including methanol, ethanol, butanol, isobutanol, and hexanol.
Preferably, the aminoplasts are melamine-, urea-, glycouril- or benzoguanamine-formaldehyde condensates etherified with an alcohol containing from one to four carbon atoms.
The aminoplast is present in the electrodepositable composition in amounts of 5 to 60, preferably 15 to 50 percent by weight based on weight of resin solids. Adjuvant quantities of the above-described polyisocyanate crosslinker, also may be present in the composition.
Usually the thermosetting composition also will contain catalysts to accelerate the cure of the crosslinking agent with reactive groups on the polymer(s). Suitable catalysts for aminoplast cure include acids such as acid phosphates and sulfonic acid or a substituted sulfonic acid. Examples include dodecylbenzene sulfonic acid, paratoluene sulfonic acid, and the like. Suitable catalysts for isocyanate cure include organotin compounds such as dibutyltin oxide, dioctyltin oxide, dibutyltin dilaurate, and the like. The catalyst is usually present in an amount of about 0.05 to about 5.0 percent by weight, preferably about 0.08 to about 2.0 percent by weight, based on the total weight of resin solids in the thermosetting composition.
Optional ingredients such as pigments can be present in the elastomeric polymer-containing compositions. Particularly suitable pigments include hiding pigments such as titanium dioxide, zinc oxide, antimony oxide, etc. and organic or inorganic UV opacifying pigments such as iron oxide, transparent red or yellow iron oxide, carbon black phthalocyanine blue, and the like. Pigments can be present in amounts of up to 35 parts by weight or less based on 100 parts by weight of total solids of the electrodepositable composition.
Other optional ingredients are anti-oxidants, UV-absorbers and hindered amine light stabilizers, such as for example, hindered phenols, benzophenones, benzotriazoles, triazoles, triazines, benzoates, piperidinyl compounds and mixtures thereof, such as those described in U.S. Pat. No. 5,260,135, which is incorporated herein by reference. These ingredients are typically added in amounts up to about 2% based on the total weight of resin solids of the electrodepositable composition. Other optional ingredients include co-solvents, coalescing aids, defoamers, plasticizers, bactericides and the like.
The aqueous cationic or anionic polymer dispersions typically are electrodeposited onto the substrate from an electrodeposition bath having a solids content of 5 to 50 percent by weight. The bath temperature is usually about 15xc2x0 C. to 35xc2x0 C. The voltage is from 100 to 400 V (load voltage) using the substrate to be coated as a cathode in the case of the cationic polymer or as an anode in the case of the anionic polymer. The film thickness of the electrodeposited coating is not particularly restricted and can vary largely depending upon the application of the finished product, etc. However, the thickness is usually between 3 to 70 microns, particularly 15 to 35 microns in terms of cured film thickness. The baking and curing temperature of the coating film is usually from 100xc2x0 C. to 250xc2x0 C. and preferably 140xc2x0 C. to 200xc2x0 C.
The electrocoating compositions are in the form of an aqueous dispersion. The term xe2x80x9cdispersionxe2x80x9d is believed to be a two-phase transparent, translucent or opaque resinous system in which the resin is in the dispersed phase and the water is in the continuous phase. The average particle size of the resinous phase is generally less than 1.0 and usually less than 0.5 microns, preferably less than 0.15 micron.
The concentration of the resinous phase in the aqueous medium is at least 1 and usually ranges from about 2 to about 60 percent by weight based on total weight of the aqueous dispersion.
In the process of applying the film coating, the aqueous dispersion of the electrodepositable composition is placed in contact with an electrically conductive anode and cathode. Upon passage of an electric current between the anode and cathode, an adherent film of the electrodepositable composition will deposit in a substantially continuous manner on either the anode or the cathode depending on whether the composition is anionically or cationically electrodepositable. Electrodeposition is usually carried out at a constant voltage in the range of from about 1 volt to several thousand volts, typically between 50 and 500 volts. Current density is usually between about 1.0 ampere and 15 amperes per square foot (10.8 to 161.5 amperes per square meter).
Additional film-forming layer(s) may be applied to the chip-resistant film-forming layer of the present invention. These layers include, without limitation, colorcoat and topcoat (clearcoat) layers. These additional layer(s) may be applied by any of the known methods for applying such coatings such as, without limitation, by spraying or dipping.